1. Field of the Invention
This invention relates generally to the driving of stepping motors and, more particularly is directed to a method of driving a stepping motor so as to make the latter suitable for effecting tracking movements of a magnetic pick up or head in respect to a magnetic disk, for example, as in an electronic still camera.
2. Description of the Prior Art
In an existing electronic still camera, a rotated magnetic disk is employed for the recording of video signals corresponding to still pictures or images in respective circular tracks on the magnetic disk. Usually, a magnetic recording and/or reproducing head is carried by an arm mounted for rectilinear movements substantially parallel to a radius of the rotated magnetic disk so that the head can be made to record and/or reproduce a video signal in a selected circular track on the disk when such selected track is traced by the head. Since the track pitch or radial distance between adjacent tracks on the magnetic disk is quite small, for example, 100 .mu.m, the movements of the arm for causing tracking of the head in respect to a selected track are minute, for example, of the order of 10 .mu.m. Therefore, when a stepping motor is employed to effect the rectilinear movements of the arm carrying the magnetic head for tracking by the latter, it is conventional to provide reduction gearing between a rotor of the stepping motor and a lead screw for propelling the rectilinearly movable arm. However, problems arise when reduction gearing is interposed between the rotor of the stepping motor and the mechanism by which rotary movements of the rotor are converted into rectilinear movements of the arm supporting the magnetic head for effecting tracking. More specifically, the presence of the reduction gearing increases the cost of the apparatus and reduces the speed with which the tracking movements of the magnetic head can be effected. Furthermore, the positioning accuracy is deteriorated due to backlash in the reduction gearing.
In order to improve the accuracy with which a driven member can be positioned by a stepping motor, a so-called micro-stepping method has been proposed for driving the stepping motor. In accordance with such micro-stepping method, a stepping motor having a rotor or driven member and a plurality of phases or windings to which input pulses are applied in predetermined sequences for establishing excitation states by which the rotor is stepped to respective positions, is operated so as to bring the rotor to rest at a desired position intermediate stepped positions which correspond to first and second excitation states, respectively. For example, if the ratio of the distances from the desired position of rest of the rotor to the stepped positions corresponding to the first and second excitation states, respectively, is 1:3, the first and second excitation states are alternately employed with a duty ratio of 3:1, that is, with a duty ratio which inversely corresponds to the ratio of the distances from the desired position of rest of the rotor to the stepped positions of the latter corresponding to the first and second excitation states, respectively. Furthermore, the frequency of the alternation between the first and second excitation states is made sufficiently high so that the rotor of the stepping motor and the system to be driven thereby, such as, the rectilinearly moveable head-supporting arm, do not oscillate or vibrate in response to the alternation of the excitation states.
However, the proposed micro-stepping method for operating or driving a stepping motor is disadvantageous in that the desired minute angular movement of the rotor of the stepping motor may be absorbed by the backlash or play in the system to be driven, for example, in the lead screw and follower pin by which the arm supporting the magnetic head is rectilinearly moved in response to turning of the rotor, so that the head is not moved at all. In other words, even if the rotor of the stepping motor is angularly displaced to extent calculated to achieve precise tracking of a selected track by the magnetic head, the actual movement of the head may not precisely correspond to that angular displacement of the rotor due to mechanical play in the motion transmitting system.
Although it has been suggested to employ a so-called uni-directional feed in which, during the microstepping operation of a stepping motor, the alternation between first and second excitation states always starts with the same one of such states, it has been found that even such uni-directional feed does not reliably achieve the accurate positioning of the head for tracking purposes due to the mechanical loss or play in the motion transmitting system.